The preparation of aromatic oligocarbonates/polycarbonates by the melt transesterification process is sufficiently well known and has been described, for example, in Encyclopedia of Polymer Science, Vol. 10 (1969), Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9, John Wiley and Sons, Inc. (1964) as well as in DE-C 10 31 512.
Because of the chemical equilibrium reaction, the transesterification process always yields products having a high content of residual monomers, i.e. of monohydroxyaryl compounds, dihydroxyaryl compounds and diaryl carbonates. Thus, in the case of polycarbonates having a low content of hydroxyl end groups, a higher content of diaryl carbonates and a lower content of monohydroxyaryl and dihydroxyaryl compounds is measured than in the case of polycarbonates having a higher content of hydroxyl end groups. In the latter case, the content of diaryl carbonates is lower but the content of monohydroxyaryl and dihydroxyaryl compounds is higher. It is further observed that the residual monomer content falls as the molecular weight increases. The residual monomers are removed from the melt in particular by degassing.
A low content of residual monomers is desirable because residual monomers lead to coatings on the molds of the processing machines during the processing of the polycarbonates. Moreover, the heat stability of residual monomers is low, so that polycarbonates having a high residual monomer content exhibit poorer properties in respect of heat stability. Furthermore, residual monomers have an adverse effect on the mechanical breaking behavior of the polycarbonate. For selected applications, such as, for example, in the foodstuffs or medical sector, residual monomers are regarded as troublesome and undesirable.
In the case of polycarbonate prepared by the melt transesterification process, the greatest proportion of residual monomers is frequently the diaryl carbonate component, especially diphenyl carbonate. It is therefore desirable when removing volatile constituents to remove these components in particular. A particularly large amount of the diaryl carbonate component forms at relatively low molecular weights at relative viscosities of from 1.18 to 1.22, as are preferably used for optical data carriers.
A further problem is the re-formation of the low molecular weight constituents, such as, for example, hydroxyaryl compounds, dihydroxyaryl compounds and carbonic acid diesters, from the polycarbonate by chemical reaction during the degassing, which makes degassing considerably more difficult.
Various processes are known for preparing polycarbonate by the melt transesterification process with subsequent removal of residual monomers by means of an additional degassing step. It is preferable in these processes to reduce the residual catalyst activity in the polycarbonate before the degassing. The reduction in the activity of the catalyst is preferably effected by the addition of acidic components as inhibitors, such as, for example, phosphoric acid, sulfuric acid, sulfurous acid, toluenesulfonic acid.
The addition and mixing in of such components that reduce the catalyst activity require an increased technical outlay. Furthermore, the mentioned components frequently have a highly corrosive action towards the materials from which the apparatus for carrying out the polymerization and degassing are usually produced.
In addition, the added components, such as, for example, phosphoric acid, may be separated from the polycarbonate with the other volatile constituents in the subsequent degassing step, accumulate in the installation and lead to damage to the installation as a result of corrosion. When the volatile constituents, including the catalyst-inhibiting component, that have been separated off are fed back into the circuit of the installation again, adverse effects on the implementation of the reaction are additionally to be expected, because the catalyst-inhibiting component can in this way inhibit the progress of the polymerization reaction.
A further problem in the preparation of polycarbonate by the melt transesterification process is that the residence time in the degassing stage under reduced partial pressure is too long. If the melt is sufficiently reactive, this may lead to a considerable increase in the molecular weight in the degassing step, which is undesirable for the degassing.
U.S. Pat. No. 5,852,156 describes a process for the preparation of polycarbonate by the melt transesterification process, in which the melt is passed under a stream of nitrogen, but not with foaming, through a zone of low pressure. The residence time in this zone is increased because the melt does not fall freely but flows downwards along vertically arranged wires. A considerable increase in the molecular weight during the degassing step is observed in this process.
EP 1 095 957 A and EP 1 095 960 A describe a process similar to that of U.S. Pat. No. 5,852,156.An inert gas is dissolved in an oligomer melt. The melt is then relieved into a zone under low pressure, with foaming. The foaming effects the removal of reaction products, so that the polymerization is able to progress. The residence time is lengthened by means of vertically arranged wires, as a result of which the molecular weight increases markedly in the degassing step. Heating of the melt before it is relieved is not provided.
EP 914 355 A describes the introduction of a separating agent of limited solubility and the subsequent relieving, optionally with foaming, of the polymer solution into a separator under low pressure.